Biological control at work: demonstrating the complementary effects of natural enemies on two contrasting pests and the damage they cause

Natural pest control is a significant service supporting agricultural production. However, the relative contribution of several functional groups of natural enemies to natural pest control is unknown for many crop-pest systems. Furthermore, the output of this regulation in terms of damage reduction is rarely assessed. In this study, based on three field experiments, we quantified the effect of ground dwelling predators and parasitoids on two functional groups of broccoli pests. Relying on physical exclusion, we showed that ground dwelling predators significantly lowered the abundance of both the cabbage root fly ( Delia radicum ) and aphids ( Brevicoryne brassicae and Myzus persicae ). Ground dwelling predators made a significant contribution to natural pest control, as they lowered pest populations by 37% on average. Parasitoids suppressed 22% of pest populations. Our results suggest that intraguild predation was not an issue since ground dwelling predators did not have a negative impact on the level of natural pest control by parasitoids. Finally, early predation by ground dwelling predators on the cabbage root fly, the most harmful pest in this study, reduced damage in a highly infested context, meaning that natural pest control can effectively support crop production. These results strongly suggest that agricultural practices limiting soil disturbances or even favoring ground dwelling predator overwintering or colonization could lead to fewer damage and losses for farmers.